Allow Flow to connect to separate DB cluster

This should be backwards compatible: default cluster value of false should load
the core database (where we currently only write data to)

With this, we can now setup Flow data on a separate database.
1 known caveat: update.php will for now always apply patches to core DB, so it
can't be used to patch Flow queries if the data lives somewhere else. We should
get that addressed some time.

Change-Id: I5ec7f3252903a54f82ee9ca4c8aada3ccf0e70c1
Merge "Allow Flow to connect to separate DB cluster"